A polyacetal resin, which is a type of engineering plastic, has, for example, excellent mechanical properties, sliding properties, frictional and abrasion properties, and chemical resistance, and has been used in a large amount as a key part for, for example, an automobile and an OA machine. It is considered that the amount of the polyacetal resin used will further increase worldwide year by year, and a further improvement of the production efficiency of the polyacetal resin is desired.
Various types of methods for producing a polyacetal resin have been known. For example, as a method for producing a polyacetal resin which is a copolymer, a general method includes the steps of: (1) producing formaldehyde using methanol as a raw material, (2) using an aqueous formaldehyde solution as a raw material, synthesizing trioxane which is to be used as a monomer, and further, for example, 1,3-dioxolane or 1,3-dioxepane which is to be used as a comonomer, and purifying the resultant products so as to have a high purity, (3) subjecting trioxane and the comonomer to polymerization reaction to synthesize a polyacetal resin, and then subjecting the synthesized resin and various additives added thereto to melting treatment to obtain product pellets, and (4) separating formaldehyde and trioxane from a dilute aqueous solution containing formaldehyde and trioxane discharged from the above steps and concentrating and recovering them using distillation or membrane separation.
Cyclic formals, such as trioxane and 1,3-dioxolane, are such unstable that they are likely to change in properties after being synthesized or purified, and they form peroxides and decomposition products, such as formaldehyde and formic acid, during the storage or transportation. The peroxides and decomposition products are disadvantageous not only in that they cause a lowering of the polymerization reaction rate in the synthesis of a polyacetal resin and a lowering of the quality of the obtained polyacetal resin, but also in that when they are formed in a large amount, insoluble paraformaldehyde may be formed and deposited. Deposition of paraformaldehyde clogs, for example, pipes and the inside of a distillation column, causing a severe problem in the stable production.
A technique has been disclosed in which crude trioxane formed in the course of obtaining high-purity trioxane is extracted with a water-insoluble inert organic solvent, and washed with an aqueous alkali solution, and then a specific tertiary amine is added to the resultant trioxane/water-insoluble inert organic solvent mixture, followed by distillation. By this technique, when trioxane is solidified and stored, or is repeatedly molten and solidified, the formation of white deposits from paraformaldehyde can be suppressed. The amount of the tertiary amine required in this technique is 0.01 to 10% by weight, based on the weight of the trioxane (see, for example, U.S. Pat. No. 3,519,650). On the other hand, a method has been disclosed in which, upon purifying crude trioxane by crystallization to obtain high-purity trioxane, an alkaline organic compound having a boiling point higher than that of trioxane is added (see, for example, Japanese Unexamined Patent Publication No. Hei 6-135957). All these methods are intended to suppress the formation of white deposits derived from paraformaldehyde when solidifying trioxane, and, for obtaining a satisfactory effect of by-product suppression, it is necessary to add an alkaline organic compound in such a high concentration that the polymerization reaction is terminated. For this reason, when the trioxane obtained by the above method is used in a polymerization reaction, the additives must be removed from the trioxane in advance so as not to inhibit the polymerization reaction.
Thus, in the conventional techniques, when trioxane in the form of a solid or flake is stored or transported, deposits derived from paraformaldehyde are caused unless an alkaline organic compound, such as a tertiary amine, in a large amount coexists with the trioxane, leading to severe problems, such as clogging of the facilities.
Further, in practice, trioxane is used in synthesizing a polyacetal resin and therefore preferably in a liquid state when being transferred to a reactor, and the trioxane in the form of a solid or flake needs much energy for heat melting and hence is not efficient. Further, a problem is encountered in the pollution caused in the step for pulverizing trioxane into a flake form. Accordingly, the trioxane obtained after synthesized or purified is preferably in a form such that the trioxane is stored or transported while maintaining the liquid state. In this connection, a method has been disclosed in which a predetermined antioxidant is added to trioxane to suppress deterioration with time caused due to the formation of impurities, such as a peroxide, formaldehyde, and formic acid, during the storage or transportation (see, for example, Japanese Patent Nos. 2908693 and 3134699). However, for example, Japanese Patent Nos. 2908693 and 3134699 have a description that the addition of an amine or phosphorus compound which terminates the polymerization reaction is inappropriate.